LONDON (Reuters) - Britain's Conservative Prime Minister David Cameron has failed to hold on to a boost in popularity which followed his decision to opt out of a new European economic pact late last year, according to the latest Reuters/Ipsos MORI Political Monitor.

February's poll of voting intentions, published on Wednesday, puts the Labour opposition in front on 41 percent, with the Conservatives on 35 percent and the Liberal Democrats on 12 percent.

Those figures reveal a 3 point drop since January for the Conservatives, senior partners in a coalition government with the smaller Liberal Democrats, taking their popularity rating to around the level it was before Cameron vetoed an accord designed to stabilise the crisit-hit euro zone.

His surprise decision had won support from Britons who are largely sceptical about closer integration with the continent.

Only four in 10 are now satisfied with Cameron's performance, putting his "net satisfaction rating" at minus 11, down 10 points from January, although the Conservative leader remains the most popular of the main three party leaders.

One reason for the decline could be the state of the economy, with nearly half of respondents thinking economic conditions will get worse over the next 12 months, although fewer think this than did in January.

The economy shrank by 0.2 percent at the end of last year, raising concerns Britain could be heading back into recession. The opposition and other critics have blamed that lacklustre performance in part on the government's austerity measures.

The government has also struggled with pushing through controversial health and welfare reforms this year.

"It has been a difficult week for the government with the continuing debate over their reforms to the NHS (state-run National Health Service) and welfare system, reflected in their drop in support," said Gideon Skinner, head of political research at Ipsos MORI.

"But during this economic gloom the public don't seem particularly enthusiastic about any of the parties."

Labour's improvement in the poll has not been matched by the party's leader Ed Miliband, who replaced Gordon Brown after the 2010 election ended 13 years of Labour rule.

Miliband's approval rating remains at its lowest ebb, a further sign that the new Labour leader has failed to galvanise his party, despite a contracting economy and fears over the social and economic impact of big government spending cuts.

With a parliamentary election not due until 2015, the coalition government is banking on a return to steady and stable economic growth over the next few years to convince voters its austerity measures were the right medicine for the economy.